The present invention relates to processes defined in the art of chemistry as free radical initiated polymerization processes, more specifically to the free radical initiated polymerization of styrene monomers. Still more specifically, it relates to such processes for the polymerization of expandable styrene polymers (xe2x80x9cexpandable polystyrenexe2x80x9d). It also relates to compositions of matter employed in such processes and to the compositions and articles of manufacture produced thereby.
The established method to produce expandable styrene polymers, generically designated as EPS, is by aqueous suspension polymerization. It is typically a batch process where two or more monomer-soluble polymerization initiators are used with a rising stepwise, continuous, or combination temperature profile. Initiators for the process are selected on the basis of their half life temperatures to provide a measured supply of radicals at selected points along the temperature profile such that effective conversion occurs within an acceptable period of time. For styrene polymerization, it is convenient to describe initiator decomposition performance in terms of one hour half life temperature, defined as that temperature sufficient to cause decomposition of one half the starting concentration of initiator over a one hour time period.
Traditionally, suspension polymerization to prepare EPS is conducted in a process using two different temperature stages and two initiators with different half life temperatures, each appropriate for the particular temperature stage. Dibenzoyl peroxide (BPO) is often used as the first stage initiator at a reaction temperature of about 82xc2x0 to 95xc2x0 C. Other first stage initiators useful in this temperature range might include tertiary butyl peroxy-2-ethylhexanoate, tertiary amyl peroxy-2-ethylhexanoate and 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane. Initiators such as tertiary butyl peroxybenzoate (TBP) or dicumyl peroxide (DCP) are widely used for the higher temperature stage at 115xc2x0 to 135xc2x0 C. The second stage is usually a finishing step intended to minimize residual monomer in the EPS. In commercial processing, this stage is often held above 125xc2x0 C. for prolonged intervals to reduce monomer content to acceptable levels.
EPS, as prepared in the suspension process, is in the form of essentially spherical beads with typical diameters of approximately 0.2 mm to 2.0 mm. In order to render the beads xe2x80x9cexpandablexe2x80x9d, it is necessary to impregnate the polymer with a blowing agent, most often low molecular weight alkane hydrocarbons like butane, 2-methylbutane, pentane and cyclohexane. EPS can be prepared in a one-step process or a two-step process. The former allows blowing agent impregnation during the polymerization and has the obvious advantage of reduced polymer handling operations. The two-step process isolates the polymer beads and segregates by size prior to a separate impregnation operation. The advantage in this case is that of precise control of bead size, a more critical parameter in some polymer molding operations. Peroxide initiator concentrations used to expedite conversion in the present invention may readily be adjusted by one of ordinary skill in the art to accommodate either process.
Characteristic shortcomings of the traditional process are long reaction times necessary to obtain adequate conversion in the first stage and relatively high finishing temperatures required in the second stage. Initiators and their use conditions described in the present invention serve to address these deficiencies. Reduced conversion time offers obvious productivity benefits. Lower finishing temperatures may offer additional process advantages such as reduced oligomer content and reduced water content in the product polymer. Oligomers may contribute undesirable polymer properties and water incorporation may cause difficulties when fabricating EPS beads into molded articles. Reducing process temperature is also an increasingly important concern as energy costs rise.
There have been previous attempts to reduce reaction times for styrene suspension polymerizations. In U.S. Pat. No. 4,029,869, Ingram teaches a suspension polymerization process using difunctional monomer additives having asymmetrical reactivity to give more desirable distributions of molecular weight. Without such additives, higher temperature styrene suspension polymerization using tertiary butyl peroxybenzoate, while reducing conversion time, yields polymer with narrow molecular weight distribution (polydispersity index), unsuitable for polymer processing. The process of the present invention requires no such monomer additives to obtain suitable polydispersity.
U.S. Pat. No. 5,266,603 teaches use of the conventional two temperature stage process using at least two peroxide initiators having lower and higher initiation temperatures to obtain low residual benzene content employing particularly specified perketal and/or monoperoxycarbonate initiators as the higher temperature initiators. The particular peroxyketals specified are not xe2x80x9cintermediatexe2x80x9d half life temperature initiators such as are contemplated by the present invention and, in particular, the peroxy groups substituted on the aliphatic or cycloaliphatic chains of the perketals are limited to t-butyl peroxy groups. Such peroxy compounds and their close analogs are known to have a half life temperature outside the range for xe2x80x9cintermediatexe2x80x9d half life initiators as contemplated by the present invention.
In the Journal of Applied Polymer Science, Vol. 50, 327-343 (1993), Hamielec notes the generally recognized fact that styrene conversion rates for suspension polymerizations can be increased by elevating initiator concentration, but this causes unacceptably low molecular weight in the typical process. To overcome this circumstance, Hamielec resorts to somewhat higher temperatures and use of a symmetrical difunctional initiator with a one hour half life temperature of approximately 98xc2x0 C. This improves conversion rate over BPO in a similar process and preserves much of the molecular weight. However, the process consumes a very substantial amount of the difunctional initiator and still appears to yield low polydispersity index. Also, no data is provided to assess effectiveness of the process to minimize residual monomer. The process of the present invention uses significantly lower concentrations of first stage initiators with one hour half life temperatures higher than that of BPO to more quickly obtain polymer of adequate molecular weight and relatively low residual monomer levels.
Glxc3xcck et al. in U.S. Pat. Nos. 5,908,272 and 6,046,245, teaches the production of expandable styrene polymers using polymerization in aqueous suspension in the presence of two peroxides which decompose at different temperatures wherein the peroxide which decomposes at the higher temperature is multifunctional.
None of the above references teach or suggest the improvements provided by the present invention to the two temperature step process for the polymerization of styrene to produce expandable polystyrene.
An object of the present invention is to employ organic peroxide initiators included in a specific half life temperature range (optionally in combination with conventional peroxide initiators) to produce expandable styrene polymers at accelerated conversion rates.
Another object of the present invention is to use these specific organic peroxide initiators to obtain EPS resin with molecular weights suitable for typical EPS applications.
Another object of the present invention is to employ these specific organic peroxide initiators in a process with significantly reduced process finishing temperature while still obtaining relatively low (less than 1000 ppm) residual monomer levels in the final polymer.
The above objects have been realized by either partially or entirely replacing conventional peroxide initiators like BPO, TBP and DCP with peroxides whose one hour half life temperatures are higher than that of BPO but lower than that of TBP. Further, pairing such peroxides, defined for purposes of the present invention as xe2x80x9cintermediatexe2x80x9d temperature initiators, with co-initiators whose half life temperature differs by 5xc2x0 to 15xc2x0 C. maintains free radical concentrations at effective levels to significantly improve conversion. This invention contemplates that half life temperature measurements are determined by measuring the rate of initiator decomposition in the aromatic solvent cumene by periodically sampling solutions of the peroxide maintained at several selected constant temperatures and determining the amount of undecomposed peroxide remaining in the sampled solution by conventional iodometric titration techniques. Such half life measurement techniques are well known by those skilled in the art. Suitable techniques for determining such half life temperatures in the same solvent using differential scanning calorimetry which provide a direct measurement of the desired half life temperature are also known to those of skill in the art and may be substituted for the iodometric measurements. The two techniques provide equivalent results for the same solvent within the expected standard experimental deviation for the procedures. It is well known in the art that half life temperatures are dependent on the solvent in which the determination is made, thus,(for precision in comparing the half life temperature of one peroxide to another, the solvent in which the half life is determined must be specified.
Thus, the invention provides in its process aspect, an improved process for the polymerization of styrene monomer to produce expandable polystyrene wherein styrene is polymerized in a process comprising the following steps:
A. heating an aqueous suspension comprising styrene monomer and at least two organic peroxide initiators, one of said organic peroxide initiators having a lower one hour half life decomposition temperature and at least one other of said organic peroxide initiators having a higher one hour half life decomposition temperature, for a time and at a temperature sufficient to effect at least partial decomposition of said lower half life organic peroxide initiator and initiate polymerization of said styrene monomer, and
B. subsequently raising the temperature of said aqueous suspension above the initial heating temperature to complete decomposition of the organic peroxide initiators in said suspension and provide complete polymerization of said styrene monomer,
wherein the improvement comprises at least one of the organic peroxide initiators incorporated in said suspension being an xe2x80x9cintermediatexe2x80x9d temperature peroxide.
Since a significant number of both first stage and finishing peroxide initiators can be utilized to enhance conversion in the present invention, a range of conversion efficiency improvements is available, depending on the specific peroxides selected and the concentrations used in the process. Peroxide selection and use levels for this process may, advantageously, be based on the desired rate of conversion enhancement consistent with heat removal capability of any specific process equipment. Such considerations, which are well understood by those experienced in practicing commercial styrene polymerization and which will depend to some extent on the requirements of the individual apparatus being utilized in such production, will avoid undesirable conditions that yield unacceptable reaction products.
The invention provides in a first composition aspect, an improved aqueous suspension suitable for polymerization to provide expandable polystyrene, said suspension comprising styrene monomer and at least two organic peroxide initiators, one of said organic peroxide initiators having a lower one hour half life decomposition temperature and at least one other of said organic peroxide initiators having a higher one hour half life decomposition temperature, wherein the improvement comprises one of the organic peroxide initiators in said suspension being an xe2x80x9cintermediatexe2x80x9d temperature peroxide.
The invention provides in a second composition aspect, expandable polystyrene produced by the process aspect of the invention from the first composition aspect of the invention and containing the decomposition products of the xe2x80x9cintermediatexe2x80x9d temperature peroxide incorporated into said first composition aspect of the invention.
The invention provides in a third composition aspect, an article of manufacture comprising a formed, at least two dimensional, object produced by shaping the second composition aspect of the invention into said formed object by conventional means.
Initiators used for effecting significant conversion enhancement in this process, as compared to the traditional process, are characterized by a specific half life temperature range which allows for higher process temperatures that notably improve the kinetics of styrene polymerization. Additionally, judicious selection of co-initiators for the process sustains such higher free radical concentrations to further increase styrene conversion rate. Thus, the combination of sufficient free radical concentrations at more favorable temperatures can dramatically enhance conversion rate over conventional, lower temperature processes.
It has been discovered that initiators with a one hour half life temperature range (as measured in the aromatic solvent cumene) of from 101xc2x0 to 111xc2x0 C. and, preferably, from 104xc2x0 to 110xc2x0 C., can substantially enhance conversion rate over typical suspension polymerization processes for styrene that use conventional first stage initiators of lower half life temperature like BPO, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane or tertiary butyl peroxy-2-ethylhexanoate. For purposes of this discussion, as stated above, it is convenient to designate such initiators with a one hour half life temperature range of from 101xc2x0 to 111xc2x0 C. as xe2x80x9cintermediatexe2x80x9d temperature peroxides.
xe2x80x9cIntermediatexe2x80x9d temperature peroxides offer a unique advantage in styrene suspension polymerization applications in that they can perform either in a capacity as first stage initiator or in a capacity as finishing initiator. Selecting which performance capacity is most advantageous is largely a function of process equipment capability. Factors such as reactor heat load tolerance and post-reactor product handling capability will certainly influence the inherent application potential of xe2x80x9cintermediatexe2x80x9d temperature peroxides.
Obtaining such significantly enhanced conversion rates relative to that obtainable with conventional initiators involves partially or completely replacing BPO (or other first stage initiators with similar half life temperature) with an xe2x80x9cintermediatexe2x80x9d temperature peroxide initiator whose one hour half life temperature is from 101xc2x0 to 111xc2x0 C., approximately 7 to 17xc2x0 C. higher than that of BPO (or other first stage initiators with similar half life temperature). These initiators are used at higher process temperatures which materially benefit conversion rates. The xe2x80x9cintermediatexe2x80x9d temperature peroxides can be utilized in amounts from 0.002 to 0.012 equivalents of initiator per liter of styrene and, more preferably, from 0.004 to 0.01 equivalents of initiator per liter of styrene.
Such xe2x80x9cintermediatexe2x80x9d temperature peroxides include, but are not limited to, 1,1,3,3-tetramethylbutyl (t-octyl) esters of alkaneperoxoic acids that are unsubstituted at the alpha position, 1,1,3,3-tetramethylbutyl (t-octyl) diesters of alkanediperoxoic acids that are unsubstituted at the alpha positions, and 1,1,3,3-tetramethylbutyl (t-octyl) esters of aroylperoxoic acids and 1,1,3,3-tetramethylbutyl (t-octyl) diesters of aroyldiperoxoic acids that are ring substituted in such manner as to result in a peroxide half life temperature within the described range. Other peroxides within the specified half life range are 1-alkoxy-1-t-alkylperoxycyclohexane, where the t-alkyl group contains 4 to 8 carbon atoms, including, without limitation, 1-alkoxy-1-t-amylperoxycyclohexane and 1-alkoxy-1-t-hexylperoxycyclohexane, where the alkoxy group contains 1 to 8 carbon atoms and the cyclohexane ring may optionally be substituted with 1 to 3 alkyl groups each, independently, having 1 to 3 carbon atoms.
Additional specific examples of these types of initiators are 1,1,3,3-tetramethylbutyl peroxyacetate (TOPA), 1,1,3,3-tetramethylbutyl peroxypropionate (TOPP), 1,1,3,3-tetramethylbutyl peroxy-3,5,5-trimethylhexanoate, di-(1,1,3,3-tetramethylbutyl)diperoxyadipate (DTODPA), 1,1,3,3-tetramethylbutyl peroxybenzoate (TOPB), di-1,1,3,3-tetramethylbutyl diperoxyterephthalate and 1-methoxy-1-t-amylperoxycyclohexane (TAPMC).
When xe2x80x9cintermediatexe2x80x9d temperature peroxides are employed as finishing initiators, it is most advantageous to select a co-initiator whose half life temperature is, approximately, 5xc2x0 to 10xc2x0 C. lower than that of the intermediate temperature peroxide used. Several non-limiting examples of useful peroxides which fall into the described half life temperature range lower than that of the xe2x80x9cintermediatexe2x80x9d temperature peroxides include tertiary butyl peroxy-2-methylpropanoate (TBPMP), tertiary amyl peroxy-2-methylpropanoate, tertiary butyl peroxycarbocyclohexane, tertiary amyl peroxycarbocyclohexane 1,4-di(tert-butylperoxycarbo)cyclohexane and 1,4-di(tert-amylperoxycarbo)cyclohexane. It will be obvious to one skilled in the art that there are numerous other peroxide initiators whose one hour half life temperatures are from about 5xc2x0 to 10xc2x0 C. lower than those of the xe2x80x9cintermediatexe2x80x9d temperature peroxides of this invention and that such other peroxides may also be similarly paired with the xe2x80x9cintermediatexe2x80x9d temperature peroxide initiators to enhance conversion rates.
If the xe2x80x9cintermediatexe2x80x9d temperature peroxide is employed as a first stage initiator, most advantage is gained by selecting a co-initiator whose half life temperature is, approximately, 5xc2x0-15xc2x0 C. higher than that of the xe2x80x9cintermediatexe2x80x9d temperature peroxide used. Such judicious use of initiator pairs with the described temperature relationship allows for more continuous generation of free radicals than is typically seen with a single, conventional first stage initiator of lower half life temperature. Also, as a consequence of higher process temperatures used with the xe2x80x9cintermediatexe2x80x9d temperature peroxides of this invention, styrene conversion kinetics further improve to notably expedite the present process relative to a traditional process.
While it is, of course, entirely possible to use high temperature peroxides like TBP and/or DCP as second stage (finishing) initiators with the xe2x80x9cintermediatexe2x80x9d temperature peroxide initiators of this invention (one hour half life temperature from 101xc2x0 to 111xc2x0 C.), such use of TBP and/or DCP may preclude using lower finishing temperatures which can, beneficially, shorten process time. A number of both tertiary butyl (t-butyl) and tertiary amyl (t-amyl) finishing peroxides fall into a desirable one hour half life temperature range between 112xc2x0 and 125xc2x0 C. However, it is generally recognized that t-amyl peroxides (i.e., organic peroxides derived from t-amyl hydroperoxide) have superior performance over t-butyl peroxides for reducing residual monomer. Thus, preference is given to t-amyl peroxyesters and t-amyl monoperoxycarbonates falling into this preferred one hour half life temperature range between 112xc2x0 and 125xc2x0 C. for EPS processes mandating minimum residual monomer levels. More specifically, tertiary amyl esters of peroxyalkanoic acids that are unsubstituted at the alpha position, t-amyl esters of aroylperoxoic acids ring substituted in such manner as to result in a peroxide half life temperature within the described range and OO-t-amyl-O-alkyl monoperoxycarbonates are preferred as finishing peroxides for the present invention. Specific examples of useful t-amyl peroxyesters are t-amyl peroxyacetate, t-amyl peroxypropionate and t-amyl peroxybenzoate. Specific examples of useful t-amyl monoperoxycarbonates are OO-t-amyl O-2-ethylhexyl monoperoxycarbonate (TAEC) and OO-t-amyl O-isopropyl monoperoxycarbonate (TAIC). Analogous higher t-alkyl derivatives (such as t-hexyl or t-heptyl) of these peroxyesters and monoperoxycarbonates would likewise be effective as finishing initiators, however, the amyl derivatives are, presently, more economical. These cited examples are illustrative and not intended to limit the scope of useful initiators for the finishing stage of the present process. Numerous combinations of finishing peroxides may also be used advantageously in the present process to reduce residual monomer level in the product polymer at lower process temperatures than would ordinarily be used with TBP and/or DCP.
Organic peroxides used as finishing initiators in the present process and having one hour half life temperatures from 112xc2x0 to 125xc2x0 C. can be utilized in amounts from 0.00 to 0.01 equivalents of peroxide initiator per liter of styrene and, more preferably, from 0.002 to 0.006 equivalents of peroxide initiator per liter of styrene.
To minimize the possibility of generating benzene as a by-product of initiator decomposition, numerous embodiments of this invention can also usefully employ initiators having no aromatic nucleus. For example, essentially any 1,1,3,3-tetramethylbutyl peroxyalkanoate unsubstituted at the alpha position could be coupled with, essentially, any OO-t-alkyl-O-alkyl monoperoxycarbonate. A specific example of such an initiator combination that would be unlikely to produce benzene as a result of initiator decomposition is TOPA and TAEC.
Styrene is the preferred monomer for the process. However, up to 15% of the weight of styrene may be replaced by other ethylenically unsaturated monomers such as alkylstyrenes, alpha methylstyrene, acrylic acid esters and acrylonitrile. The process of the invention can be used with styrene to water ratios which can, typically, vary from about 0.3 to 1.5 parts by weight styrene per 1.0 part by weight water.
Other common and useful additives for the present suspension process include inorganic suspension stabilizers like calcium phosphate or magnesium pyrophosphate, organic suspension stabilizers like polyvinylpyrrolidone, polyvinyl alcohol or hydroxyethylcellulose, surfactants, blowing agents, chain transfer agents, nucleating agents, expansion aids, lubricants and plasticizers. Halogenated organic compounds (such as hexabromocyclododecane) are also particularly useful as flame retardant additives in this process. Such halogenated organic compounds are usually employed together with free radical generating synergists like bicumyl and dicumyl peroxide (DCP). Lower finishing temperatures, obtainable in specific embodiments of the present process, advantageously limit the degradation of such synergists. Blowing agents can be added before or at any time during the polymerization in amounts of up to 10 weight percent based on weight of charged monomer. Also useful for the present process is conducting the polymerization in the presence of up to 10 weight percent (based on monomer) of finely divided graphite particles using procedures similar to those described in U.S. Pat. No. 6,046,245 and references cited therein.
It is recognized that supplemental amounts of other peroxides with one hour half life temperatures between 80xc2x0 and 125xc2x0 C. may, optionally, be used to advantageously modify molecular weight and/or molecular weight distribution (polydispersity index) of the resulting polymer without substantial effect on the conversion enhancement inherent to the present process.
Measurement of molecular weight, molecular weight averages and the distribution of molecular weights (polydispersity) are well known in the art. U.S. Pat. No. 4,777,230, for example provides a discussion of the techniques in relation to acrylic coatings where narrow polydispersity is desired. One of skill in the art will understand that with suitable modification for the desired difference in polydispersity of the styrene polymers contemplated as being produced by the present invention, the general principles in that discussion will apply herein.